The present invention relates to communications systems applying slow frequency hopping, and more particularly to a procedure for synchronizing two frequency hopping units to each other in order to establish a communication link.
Frequency hop (FH) spreading has been an attractive communications form in military applications for a long time. By sending signals sequentially in different parts of the radio spectrum in a pseudo-random way, both high security against eavesdropping, and immunity against narrowband interferers is obtained. With the advent of fast, cheap, and low-power synthesizers, FH transceivers are becoming commercially attractive, and are used more and more in civil applications as well. For certain wireless radio systems, FH is especially attractive because of its immunity to unknown interference and to Rayleigh fading. Examples are radio systems using unlicensed bands like the Industrial, Scientific and Medical (ISM) bands at 900, 2400 and 5700 MHz. Because the radio communications are unregulated in these bands (apart from some transmission power restrictions), communication systems using this band must be capable of sustaining any (i.e., a priori unknown) interference. FH appears to be an attractive tool in fighting the interference.
Two types of FH systems can be distinguished: slow FH and fast FH. In slow-FH communications, a burst of symbols is transmitted in a hop; thus, the symbol rate is higher than the hop rate. In fast-FH, a single symbol is spread over several hops, so that the hop rate is higher than the symbol rate. Fast-FH puts high requirements on the speed of the transceiver electronics, especially at higher symbol rates. Therefore fast-FH is not attractive for portable usage because of higher power consumption. Slow-FH provides all the system features required in a wireless communications system, that is, interference immunity and fading immunity.
For a FH connection to operate, synchronization between the two hopping transceivers is required: the transmission (TX) hop of one unit must be the receive (RX) hop of the other unit, and vice versa. Once the two units are locked, they just use the same hop sequence at the proper rate in order to maintain the connection. However, a problem is to get the two units synchronized initially. When there is no connection, a portable unit is usually in a standby mode. In this mode, it sleeps most of the time, but periodically it wakes up to listen for paging messages from units that want to connect. A problem with a FH scheme is that the paging unit does not know when and on what hop channel the unit in standby will listen for paging messages. This results in an uncertainty both in time and in frequency.
Conventional techniques have attempted to solve the problem of establishing a connection between a paging unit and a unit in standby mode. In U.S. Pat. No. 5,353,341 issued to Gillis, a single reserved hop channel is used for access. The paging unit always sends paging messages out on this single reserved channel, and when the standby unit periodically wakes up, it only monitors the one reserved channel. Because there is no hopping of the access channel, there is no frequency uncertainty. However, this strategy has the drawback of lacking the benefits that an FH strategy can provide: When the reserved channel is disturbed by a jammer, no access can take place.
U.S. Pat. No. 5,430,775 to Fulghum et al. discloses a system in which reserved channels are used as agreed upon by sender and recipient. In this case, there are two reserved channels: one to "reserve" the access channel, and the other is the access channel itself. The access process lacks the benefits that FH can provide because both the reservation and the access channel do not hop, but are instead constant.
U.S. Pat. No. 5,528,623 to Foster, Jr. discloses a system in which both the sender and the recipient hop in the access procedure, thereby providing the full benefits of a FH scheme. However, in this system the recipient is required to hop quickly during the wake-up period, while the paging unit hops slowly. As a result, this system has the undesirable effect of requiring the recipient (i.e., the unit in standby) to expend a relatively large amount of power during every wake-up period, just to check to see whether it is being paged. Another apparent shortcoming of the system as described by Foster, Jr. is that there is no explanation of how the return message from the recipient to the sender is arranged. That is, a 3.3 ms return period is defined in which the sender listens for a response; but upon receipt of the page message, the recipient does not know when this 3.3 ms listening period starts.